1. Field of the Invention
This invention relates to a heat-developable photosensitive material, and more particularly to a heat-developable photosensitive material whose thermal fogging during development is markedly reduced without detriment to image characteristics, such as sensitivity loss, by adding an aliphatic dicarboxylic acid having at least four carbon atoms.
2. Description of the Prior Art
Although conventional silver halide photography is excellent in photosensitivity, gradient and the like, as compared with other photographies, its handling is complicated and it is unfavorable for the mechanization of processing steps in many respects because its processing steps such as fixation and bleaching ae of wet process. A number of attempts have been made to form an image by using a dry process instead of the silver halide photography. A process for forming an image by using a heat-developable photosensitive material was proposed as the one which was particulrly excellent as compared with the conventional image formation processes.
For example, Japanese Patent Publication No. 4924/1968 described a heat-developable photosensitive material comprising a silver halide in catalytic contact with an organosilver salt. Similar materials were described in Japanese Patent Publication No. 26582/1969 and Japanese Patent Laid-Open No. 6074/1971. In addition, there have been proposed heat-developable photosensitive materials which are used in such a way that they are activated and rendered photosensitive by a heat treatment before exposure and they are totally heated in order to form images after exposure, and processes for preparing the same. These heat-developable photosensitive materials comprise a heat-developable photosensitive element either containing no silver halide or containing silver halide but having no photosensitivity, as described in Japanese Patent Publication Nos. 29819/1976, 41967/1978 and 5687/1979.
Although the heat-developable photosensitive materials have an advantage that their image formation can be performed without resort to a wet process, their image characteristics such as sensitivity and fog density can not be though to be sufficient yet as compared with those of the conventional silver halide photography. Particularly, thermal fogging is prone to occur at the unexposed area (background area) during heat development, so that the latitudes of a development temperature and a development time become a great practical problem. In order to control an increase in the thermal fogging, a variety of processes have been proposed. Japanese Patent Publication No. 11113/1972 proposed the addition of mercury compounds as a thermal fogging preventive. Although this process can provide an excellent thermal fogging preventing effect, it is not preferable because of the toxicity of the compound. Moreover, there is a drawback that an unexposed photographic material loses its sensitivity during storage. In addition to the mercury compounds, a variety of antifoggants are proposed. For example, Japanese Patent Laid-Open No. 54428/1976 proposed the use of a combination of sulphinic acids with phthalic acids, but this process is not desirable because it has a problem that coloration occurs by light after the image formation. Moreover, processes were proposed which comprises a variety of compounds, such as thiouracils in Japanese Patent Laid-Open No. 3223/1976, carboxyl group- or sulfonic group-containing polymer acids in Japanese Patent Laid-Open No. 104338/1976, thiosulfonic acids in Japanese Patent Publication No. 44212/1979, or benzoic acids in Japanese Patent Laid-Open No. 125015/1978, but none of these processes could provide an antifogging effect which was superior to that of the mercury compounds which were undesirable because of their toxicity.